xref: /openbmc/linux/net/ipv4/tcp_vegas.c (revision d7a3d85e)
1 /*
2  * TCP Vegas congestion control
3  *
4  * This is based on the congestion detection/avoidance scheme described in
5  *    Lawrence S. Brakmo and Larry L. Peterson.
6  *    "TCP Vegas: End to end congestion avoidance on a global internet."
7  *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
8  *    October 1995. Available from:
9  *	ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
10  *
11  * See http://www.cs.arizona.edu/xkernel/ for their implementation.
12  * The main aspects that distinguish this implementation from the
13  * Arizona Vegas implementation are:
14  *   o We do not change the loss detection or recovery mechanisms of
15  *     Linux in any way. Linux already recovers from losses quite well,
16  *     using fine-grained timers, NewReno, and FACK.
17  *   o To avoid the performance penalty imposed by increasing cwnd
18  *     only every-other RTT during slow start, we increase during
19  *     every RTT during slow start, just like Reno.
20  *   o Largely to allow continuous cwnd growth during slow start,
21  *     we use the rate at which ACKs come back as the "actual"
22  *     rate, rather than the rate at which data is sent.
23  *   o To speed convergence to the right rate, we set the cwnd
24  *     to achieve the right ("actual") rate when we exit slow start.
25  *   o To filter out the noise caused by delayed ACKs, we use the
26  *     minimum RTT sample observed during the last RTT to calculate
27  *     the actual rate.
28  *   o When the sender re-starts from idle, it waits until it has
29  *     received ACKs for an entire flight of new data before making
30  *     a cwnd adjustment decision. The original Vegas implementation
31  *     assumed senders never went idle.
32  */
33 
34 #include <linux/mm.h>
35 #include <linux/module.h>
36 #include <linux/skbuff.h>
37 #include <linux/inet_diag.h>
38 
39 #include <net/tcp.h>
40 
41 #include "tcp_vegas.h"
42 
43 static int alpha = 2;
44 static int beta  = 4;
45 static int gamma = 1;
46 
47 module_param(alpha, int, 0644);
48 MODULE_PARM_DESC(alpha, "lower bound of packets in network");
49 module_param(beta, int, 0644);
50 MODULE_PARM_DESC(beta, "upper bound of packets in network");
51 module_param(gamma, int, 0644);
52 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
53 
54 /* There are several situations when we must "re-start" Vegas:
55  *
56  *  o when a connection is established
57  *  o after an RTO
58  *  o after fast recovery
59  *  o when we send a packet and there is no outstanding
60  *    unacknowledged data (restarting an idle connection)
61  *
62  * In these circumstances we cannot do a Vegas calculation at the
63  * end of the first RTT, because any calculation we do is using
64  * stale info -- both the saved cwnd and congestion feedback are
65  * stale.
66  *
67  * Instead we must wait until the completion of an RTT during
68  * which we actually receive ACKs.
69  */
70 static void vegas_enable(struct sock *sk)
71 {
72 	const struct tcp_sock *tp = tcp_sk(sk);
73 	struct vegas *vegas = inet_csk_ca(sk);
74 
75 	/* Begin taking Vegas samples next time we send something. */
76 	vegas->doing_vegas_now = 1;
77 
78 	/* Set the beginning of the next send window. */
79 	vegas->beg_snd_nxt = tp->snd_nxt;
80 
81 	vegas->cntRTT = 0;
82 	vegas->minRTT = 0x7fffffff;
83 }
84 
85 /* Stop taking Vegas samples for now. */
86 static inline void vegas_disable(struct sock *sk)
87 {
88 	struct vegas *vegas = inet_csk_ca(sk);
89 
90 	vegas->doing_vegas_now = 0;
91 }
92 
93 void tcp_vegas_init(struct sock *sk)
94 {
95 	struct vegas *vegas = inet_csk_ca(sk);
96 
97 	vegas->baseRTT = 0x7fffffff;
98 	vegas_enable(sk);
99 }
100 EXPORT_SYMBOL_GPL(tcp_vegas_init);
101 
102 /* Do RTT sampling needed for Vegas.
103  * Basically we:
104  *   o min-filter RTT samples from within an RTT to get the current
105  *     propagation delay + queuing delay (we are min-filtering to try to
106  *     avoid the effects of delayed ACKs)
107  *   o min-filter RTT samples from a much longer window (forever for now)
108  *     to find the propagation delay (baseRTT)
109  */
110 void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us)
111 {
112 	struct vegas *vegas = inet_csk_ca(sk);
113 	u32 vrtt;
114 
115 	if (rtt_us < 0)
116 		return;
117 
118 	/* Never allow zero rtt or baseRTT */
119 	vrtt = rtt_us + 1;
120 
121 	/* Filter to find propagation delay: */
122 	if (vrtt < vegas->baseRTT)
123 		vegas->baseRTT = vrtt;
124 
125 	/* Find the min RTT during the last RTT to find
126 	 * the current prop. delay + queuing delay:
127 	 */
128 	vegas->minRTT = min(vegas->minRTT, vrtt);
129 	vegas->cntRTT++;
130 }
131 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);
132 
133 void tcp_vegas_state(struct sock *sk, u8 ca_state)
134 {
135 	if (ca_state == TCP_CA_Open)
136 		vegas_enable(sk);
137 	else
138 		vegas_disable(sk);
139 }
140 EXPORT_SYMBOL_GPL(tcp_vegas_state);
141 
142 /*
143  * If the connection is idle and we are restarting,
144  * then we don't want to do any Vegas calculations
145  * until we get fresh RTT samples.  So when we
146  * restart, we reset our Vegas state to a clean
147  * slate. After we get acks for this flight of
148  * packets, _then_ we can make Vegas calculations
149  * again.
150  */
151 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
152 {
153 	if (event == CA_EVENT_CWND_RESTART ||
154 	    event == CA_EVENT_TX_START)
155 		tcp_vegas_init(sk);
156 }
157 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
158 
159 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
160 {
161 	return  min(tp->snd_ssthresh, tp->snd_cwnd-1);
162 }
163 
164 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
165 {
166 	struct tcp_sock *tp = tcp_sk(sk);
167 	struct vegas *vegas = inet_csk_ca(sk);
168 
169 	if (!vegas->doing_vegas_now) {
170 		tcp_reno_cong_avoid(sk, ack, acked);
171 		return;
172 	}
173 
174 	if (after(ack, vegas->beg_snd_nxt)) {
175 		/* Do the Vegas once-per-RTT cwnd adjustment. */
176 
177 		/* Save the extent of the current window so we can use this
178 		 * at the end of the next RTT.
179 		 */
180 		vegas->beg_snd_nxt  = tp->snd_nxt;
181 
182 		/* We do the Vegas calculations only if we got enough RTT
183 		 * samples that we can be reasonably sure that we got
184 		 * at least one RTT sample that wasn't from a delayed ACK.
185 		 * If we only had 2 samples total,
186 		 * then that means we're getting only 1 ACK per RTT, which
187 		 * means they're almost certainly delayed ACKs.
188 		 * If  we have 3 samples, we should be OK.
189 		 */
190 
191 		if (vegas->cntRTT <= 2) {
192 			/* We don't have enough RTT samples to do the Vegas
193 			 * calculation, so we'll behave like Reno.
194 			 */
195 			tcp_reno_cong_avoid(sk, ack, acked);
196 		} else {
197 			u32 rtt, diff;
198 			u64 target_cwnd;
199 
200 			/* We have enough RTT samples, so, using the Vegas
201 			 * algorithm, we determine if we should increase or
202 			 * decrease cwnd, and by how much.
203 			 */
204 
205 			/* Pluck out the RTT we are using for the Vegas
206 			 * calculations. This is the min RTT seen during the
207 			 * last RTT. Taking the min filters out the effects
208 			 * of delayed ACKs, at the cost of noticing congestion
209 			 * a bit later.
210 			 */
211 			rtt = vegas->minRTT;
212 
213 			/* Calculate the cwnd we should have, if we weren't
214 			 * going too fast.
215 			 *
216 			 * This is:
217 			 *     (actual rate in segments) * baseRTT
218 			 */
219 			target_cwnd = (u64)tp->snd_cwnd * vegas->baseRTT;
220 			do_div(target_cwnd, rtt);
221 
222 			/* Calculate the difference between the window we had,
223 			 * and the window we would like to have. This quantity
224 			 * is the "Diff" from the Arizona Vegas papers.
225 			 */
226 			diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT;
227 
228 			if (diff > gamma && tp->snd_cwnd <= tp->snd_ssthresh) {
229 				/* Going too fast. Time to slow down
230 				 * and switch to congestion avoidance.
231 				 */
232 
233 				/* Set cwnd to match the actual rate
234 				 * exactly:
235 				 *   cwnd = (actual rate) * baseRTT
236 				 * Then we add 1 because the integer
237 				 * truncation robs us of full link
238 				 * utilization.
239 				 */
240 				tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1);
241 				tp->snd_ssthresh = tcp_vegas_ssthresh(tp);
242 
243 			} else if (tp->snd_cwnd <= tp->snd_ssthresh) {
244 				/* Slow start.  */
245 				tcp_slow_start(tp, acked);
246 			} else {
247 				/* Congestion avoidance. */
248 
249 				/* Figure out where we would like cwnd
250 				 * to be.
251 				 */
252 				if (diff > beta) {
253 					/* The old window was too fast, so
254 					 * we slow down.
255 					 */
256 					tp->snd_cwnd--;
257 					tp->snd_ssthresh
258 						= tcp_vegas_ssthresh(tp);
259 				} else if (diff < alpha) {
260 					/* We don't have enough extra packets
261 					 * in the network, so speed up.
262 					 */
263 					tp->snd_cwnd++;
264 				} else {
265 					/* Sending just as fast as we
266 					 * should be.
267 					 */
268 				}
269 			}
270 
271 			if (tp->snd_cwnd < 2)
272 				tp->snd_cwnd = 2;
273 			else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
274 				tp->snd_cwnd = tp->snd_cwnd_clamp;
275 
276 			tp->snd_ssthresh = tcp_current_ssthresh(sk);
277 		}
278 
279 		/* Wipe the slate clean for the next RTT. */
280 		vegas->cntRTT = 0;
281 		vegas->minRTT = 0x7fffffff;
282 	}
283 	/* Use normal slow start */
284 	else if (tp->snd_cwnd <= tp->snd_ssthresh)
285 		tcp_slow_start(tp, acked);
286 }
287 
288 /* Extract info for Tcp socket info provided via netlink. */
289 size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
290 			  union tcp_cc_info *info)
291 {
292 	const struct vegas *ca = inet_csk_ca(sk);
293 
294 	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
295 		info->vegas.tcpv_enabled = ca->doing_vegas_now,
296 		info->vegas.tcpv_rttcnt = ca->cntRTT,
297 		info->vegas.tcpv_rtt = ca->baseRTT,
298 		info->vegas.tcpv_minrtt = ca->minRTT,
299 
300 		*attr = INET_DIAG_VEGASINFO;
301 		return sizeof(struct tcpvegas_info);
302 	}
303 	return 0;
304 }
305 EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
306 
307 static struct tcp_congestion_ops tcp_vegas __read_mostly = {
308 	.init		= tcp_vegas_init,
309 	.ssthresh	= tcp_reno_ssthresh,
310 	.cong_avoid	= tcp_vegas_cong_avoid,
311 	.pkts_acked	= tcp_vegas_pkts_acked,
312 	.set_state	= tcp_vegas_state,
313 	.cwnd_event	= tcp_vegas_cwnd_event,
314 	.get_info	= tcp_vegas_get_info,
315 
316 	.owner		= THIS_MODULE,
317 	.name		= "vegas",
318 };
319 
320 static int __init tcp_vegas_register(void)
321 {
322 	BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
323 	tcp_register_congestion_control(&tcp_vegas);
324 	return 0;
325 }
326 
327 static void __exit tcp_vegas_unregister(void)
328 {
329 	tcp_unregister_congestion_control(&tcp_vegas);
330 }
331 
332 module_init(tcp_vegas_register);
333 module_exit(tcp_vegas_unregister);
334 
335 MODULE_AUTHOR("Stephen Hemminger");
336 MODULE_LICENSE("GPL");
337 MODULE_DESCRIPTION("TCP Vegas");
338